The present invention relates to a flash memory device, and more particularly to a method for setting a programming start bias for a NAND flash memory device using an Incremental Step Pulse Programming (ISPP) scheme and a method for programming a NAND flash memory device using the programming start bias setting method.
Generally, a NAND flash memory includes a string of cells connected in series. The string may include one or more string selection transistors. An operation for programming and/or erasing a NAND flash memory device is performed through tunneling such as Flower-Nordheim (F-N) tunneling. Specifically, the operation for programming a NAND flash memory device uses coupling between the gate and channel. For example, a cell that is to be programmed has a relatively large voltage difference between the gate and channel while a cell that is not to be programmed has a relatively small voltage difference between the gate and channel. The operation for programming a NAND flash memory device also involves a cell threshold voltage distribution.
Generally, the cell threshold voltage distribution is adjusted using an Incremental Step Pulse Programming (ISPP) scheme. According to the general ISPP scheme, programming is performed by sequentially applying biases, beginning with a start bias which is incremented in steps of ΔV, as shown in FIG. 1. That is, programming is first performed with the first bias VISPP1 as the start bias and is then performed with the second bias VISPP2 obtained by increasing the first bias VISPP1 by ΔV. This process is repeated until the last bias VISPPn is applied. A programming verification process is performed with a relatively low verification bias Vverify between each programming period. It is known in the art that performing such an ISPP programming suppresses the occurrence of over-programming. Over-programming is a phenomenon in which a read operation of a cell fails because part of the programmed cell threshold voltage distribution exceeds the read voltage.
However, using the ISPP scheme, it is not possible to avoid widening of the cell threshold voltage distribution as shown in FIG. 2 for a variety of reasons. Specifically, cell threshold voltage distributions 220, 230, and 240, which are widened compared to the ideal cell threshold voltage distribution 210, may be due to parasitic effects such as verify vs. read offset, programming speed, back pattern dependency, and floating gate coupling. In addition, the cell threshold voltage distribution may move to the right due to programming and erasure cycling. The widening or right movement of the cell threshold voltage distribution may cause an over-programming phenomenon, resulting in a failure of the device.